Re: [Rd] dealing with empty actual arguments matched by '...' formals

Duncan Murdoch <murdoch@math.aau.dk> writes:

> Tony Plate wrote:
> > I'm trying to write some functions to deal with empty actual
> > arguments that are picked up by '...' formals. Such actual
> > arguments are common (and very useful) in calls to subsetting
> > functions, e.g., x[1:2,]. It seems that R and S-PLUS treat these
> > arguments differently: in S-PLUS list(...) will return a list
> > containing just the non-empty arguments, whereas in R list(...)
> > stops with an error:
> > > # In R:
> > > f <- function(x, ...) list(...)
> > > f(1,2)
> > [[1]]
> > [1] 2
> > > f(1,2,)
> > Error in f(1, 2, ) : argument is missing, with no default
> > >
> > So it seems that quite different methods must be used in S-PLUS and
> > R to detect and process the arguments of a function that can have
> > empty arguments matched to '...'.
>
> Can you give an example where it's useful to do this, i.e. to have a
> call like f(1,2,)? I've never used that construction as far as I can
> recall.

The standard case is indexing, as Tony mentions.

The whole thing is somewhat tricky because at least some of R's semantics are deliberately different from S. E.g.

> f <- function(i) g(i)
> g <- function(i) missing(i)
> f()

[1] TRUE Same thing in S gives FALSE. S looks at the call to g whereas R looks at the value. This works by passing a "magic bullet" which is implemented as the "empty name", as you can get to see by doing something like

> f <- function(...) match.call(expand.dots=FALSE)$...
> l <- f(1,,2)
> eval(l[[2]])

Error in eval(expr, envir, enclos) : Argument is missing, with no default
> mode(l[[2]])

[1] "name"
> as.character(l[[2]])

[1] ""

One side effect of R's way of doing things is that a call to list(i,j,k) with k missing is hard to tell from list(i,j,). However, list() must be doing that somehow... I'm not sure it is a good thing, but it may have been necessary for S compatibility.

I think that what Tony was up to might be doable through variations on the match.call() scheme above.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907

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